A sheathed thermocouple or a sheathed resistance temperature detector which are sheath type temperature sensors, is connected to a cable for leading a measurement signal to a receiver, that is to say a compensating lead cable is connected to the sheathed thermocouple, and an extension cable is connected to the sheathed resistance temperature detector. One type of mechanism for making these connections is a terminal head (also referred to as a terminal box), to which the present invention is directed.
FIG. 4 shows the connection relationship between sheath type temperature sensors and cables. FIG. 4(a) shows the connection relationship between a sheathed thermocouple 301 and a compensating lead cable 701, and FIG. 4(b) shows the connection relationship between a sheathed resistance temperature detector 302 and an extension cable 702. The portion enclosed with a dashed line is the connection portion in the terminal head. Note that although FIG. 4 shows cross-sectional diagrams, external forms are shown for a coating 71 on the compensating lead cable 701 and the extension cable 702, insulating coatings 72 on compensating leads 731 of the compensating lead cable 701 and leads 732 of the extension cable 702, and a resistance temperature detector element 35. Also, portions with the same functions in FIGS. 4(a) and 4(b) are denoted by the same reference numerals.
In the sheathed thermocouple 301, one pair of thermocouple element wires 31a is housed in a metal sheath 32 via inorganic insulating powder 33 such as magnesia or alumina. In the terminal head, the thermocouple element wires 31a are connected to compensating leads 731 that are exposed from respective insulating coatings 72 and a coating 71 of the compensating lead cable 701.
In the sheathed resistance temperature detector 302, a resistance temperature detector element 35 and leads 31b thereof are housed inside a metal sheath 32 via inorganic insulating powder 33 such as magnesia or alumina. In the terminal head, the leads 31b of the resistance temperature detector element 35 are connected to leads 732 that are exposed from respective insulating coatings 72 and a coating 71 of the extension cable 702. As is widely known, temperature measurement is performed by the resistance temperature detector element 35 using a two wire, three wire, or four wire method depending on the number of lead 31b branches from the resistance temperature detector element 35, and FIG. 4(b) shows the case where the three wire method is used.
In both the sheathed thermocouple 301 and the sheathed resistance temperature detector 302, the end of the sheath 32 is provided with a resin seal 34 so as to prevent the generation of temperature measurement error due to a reduction in the insulation resistance of the inorganic insulating powder 33 brought about by the entrance of moisture.
The general structure of a conventional terminal head will now be described with reference to FIG. 5, which shows the terminal head of a sheathed thermocouple. FIG. 5(a) is a cross-sectional diagram of a terminal head 10. FIG. 5(b) is a view along arrows C-C in FIG. 5(a), and FIG. 5(c) is a cross-section taken along D-D in FIG. 5(b). The external form of the left half of a cover 102 is shown in FIG. 5(a).
An outer frame body 101 has an upper opening to which the cover 102 is screwed, a side opening 104, and a bottom opening 105, and the cover 102 is connected to the outer frame body 101 by a chain 103 in order to prevent loss.
Inside the outer frame body 101, a terminal plate 20 made of an electrical insulating material is fixed to the outer frame body 101 via multiple terminal plate fixing screws 201. Two terminal fittings 202 are fixed to the terminal plate 20 by terminal fitting fixing screws 207, and a sensor wiring terminal 206 and a cable wiring terminal 203 are attached to each of the terminal fittings 202. These two types of terminals in each pair are electrically conducted to each other due to the terminal fittings 202.
Also, a sheath attachment fitting 208 is attached to the terminal plate 20 by sheath attachment fitting firing screws 205, a sheath 32 of the sheathed thermocouple 301, which has been inserted through the bottom opening 105 of the outer frame body 101, is welded to the sheath attachment fitting 208, and thus the position of the sheathed thermocouple 301 is fixed by the sheath attachment fitting 208.
The end of the sheathed thermocouple 301 is provided with a resin seal 34, insulating tubes 312 cover the pair of thermocouple element wires 31a protruding from the sheath 32 in order to prevent contact between them, and the tips of the thermocouple element wires 31a are connected to the sensor wiring terminals 206.
A protective tube 106 for preventing deformation of and damage to the sheathed thermocouple 301 is fixed to the bottom opening 105 of the outer frame body 101 by screwing together a locknut 107 and the outer frame body 101.
A compensating lead cable from a receiver (not shown in FIG. 5) is inserted through the side opening 104 of the outer frame body 101, and the compensating leads thereof are connected to the cable wiring terminals 203.
The sheathed thermocouple 301 and the compensating lead cable are connected as described above. Note that the number of sets of a sensor wiring terminal 206, a cable wiring terminal 203, and a terminal fitting 202 that is needed is the same as the number of leads that the sheath type temperature sensor has. There are two sets in the case of the sheathed thermocouple 301 housing one pair of thermocouple element wires 31a as shown in FIG. 5, but there are four sets in the case of a double element type sheathed thermocouple that houses two pairs of thermocouple element wires, and there are two sets, three sets, and four sets respectively in the above-described cases of two wire, three wire, and four wire type sheathed resistance temperature detectors.
Also, in the case of the sheathed resistance temperature detector 302, the thermocouple element wires 31a correspond to the leads 31b, the compensating lead cable 701 corresponds to the extension cable 702, and the compensating leads 731 correspond to the leads 732 of the extension cable as shown in FIG. 4.
In the following, the sheathed thermocouple 301 and the sheathed resistance temperature detector 302 are collectively referred to as a sheath type temperature sensor, the thermocouple element wires 31a and the leads 31b are collectively referred to as sheath type temperature sensor leads, the compensating lead cable 701 and the extension cable 702 are collectively referred to as a cable, and the compensating leads 731 of the compensating lead cable 701 and the leads 732 of the extension cable 702 are collectively referred to as cable leads.
Besides the shape shown in FIG. 5, examples of the shape of a terminal head include the shapes shown in FIG. 1 of Patent Document 1, FIG. 2 of Patent Document 2. FIGS. 1 and 4 of Patent Document 3, and FIG. 4 of Patent Document 4. However, the basic structure is the same as that in FIG. 5 in all of these cases, that is to say an outer frame body with a cover (referred to as a body in Patent Document 1, a terminal box in Patent Document 2, an outer frame body in Patent Document 3, and a terminal housing in Patent Document 4) is internally provided with a terminal plate (referred to as a terminal block in Patent Document 1, a terminal plate in Patent Documents 2 and 3, and a terminal board in Patent Document 4) that is fixed to the outer frame body, and wiring terminals (referred to as terminal fittings in Patent Document 1, terminal blocks in Patent Document 2, wiring screws in Patent Document 3, and connection terminals or terminals in Patent Document 4) are provided on the terminal plate. Also, all of these cases are the same in that a sheath type temperature sensor inserted through a bottom opening in the outer frame body is connected to a cable inserted through a side opening, using the wiring terminals as junctions.